Capacitive sensors are employed to a wide extent in measuring technology and sensorics. Exemplarily, distances between two measuring points can be determined when precisely measuring the capacitance between the two measuring points so that when knowing the theoretical context between capacitance and distance, the distance between the two measuring points can be deduced using the capacitance measured. In general, the capacitance between two surfaces is determined by the surface geometry and a dielectric surrounding the surfaces. If the characteristics of the dielectric are changed by bringing a material having different dielectric characteristics close to the surfaces, the capacitance between the two surfaces sometimes varies considerably.
Many technological applications make use of this by utilizing a variation in capacitance to prove contacting of an object or a surface. This is, for example, the case when driving special touch-sensitive displays. In particular, touching the frame of a car window by a part of the human body can be proved by means of capacitive measurements, wherein the capacitance between two wires integrated into the sealing rubber or the capacitance between a single wire and the metallic window frame is exemplarily determined. This allows implementing a reliable protection against getting trapped which prevents an electric window lift from closing the car window when a part of the body touches the sealing area or is close thereto so that serious injuries are avoided. The capacitive measurement here has the great advantage that, compared to conventional methods which are based on an increase in the motor current when the window hits an obstacle, it is considerably securer since no mechanical contact between the window and the part of the body is necessary for the method to work. With mechanical contact, a comparatively small force causing a small variation in current below the regulating threshold may eventually already cause injuries, like for example in a child's hand. Electric motor tracking may also result in trapped parts of the body being injured although the trapping in principle has been recognized. The problems mentioned above are in principle prevented by the capacitive measurement.
A number of measuring methods are known to allow precise measurements of small variations in capacitance.
Exemplarily, detunable oscillators (excited RCL circuits) where the resonant frequency is influenced by a varying capacitance are used. Thus, the voltage across an ohmic resistor R which with a fixed resistance R and a fixed inductance L is proportional to the capacitance, is usually determined as measuring quantity. Normally, the voltage measured then has to be digitalized to calculate the capacitance from the proportionality relation.
Furthermore, charge transfer methods where a first capacitance is charged in a first phase and the charge is transformed to a second capacitance in a second phase are conventional methods. Here, both the first and the second capacitances may be used as measuring capacitance. The quantity of the measuring capacitance must be known in order to be able to determine the capacitance of the capacitor to be measured. Usually, the voltage across the measuring capacitance is determined as measuring quantity.
Bridge circuits where the capacitance to be measured is determined by a time-consuming tuning method in which usually a diagonal voltage of the bridge circuit is regulated to be zero are frequently used for measuring capacitances.
Additionally, synchronous demodulator methods may be employed for measuring capacitances.
Since the analog measuring signals are typically digitalized for further signal processing, often the problem arises that the output voltage range provided by the analog sensor (such as, for example, a capacitance) does not match the dynamic input range of a downstream analog-to-digital converter stage, resulting in a decrease in precision in the digital measuring result.
U.S. Pat. No. 6,452,521 B1 describes a concept of how the dynamic range of an analog measuring signal at the output of a sensor can be mapped or adjusted to the dynamic input range of a delta-sigma modulator. In order to achieve this, a mapping circuit is coupled to the integrator of a delta-sigma modulator to adjusting the analog input range of the integrator to the analog output range of the sensor. Thus, the integrator provides an integrated output signal to a controller which produces a digital output signal, the digital output signal being in a digital range of values representing the potential range of values of the analog input signal.
Capacitance measuring methods used so far are based on analog circuits the measuring signals of which must be processed using complicated analog signal processing or be transmitted in an analog manner to an analog-to-digital converter in order to allow subsequent digital processing. The great number of electrical devices necessary for such an implementation is, on the one hand, of disadvantage concerning the cost caused. On the other hand, the result is increased space requirements in the implementation, which is also of disadvantage when only little space is available, like for example when installing capacitance measuring circuits in a vehicle.
The temporal behavior of the measuring circuit additionally has an important role in monitoring tasks. Detunable oscillators, for example, first have to settle at a new frequency before a reliable measurement may take place, wherein the subsequent analog-to-digital conversion necessitates additional time so that a reliable measuring result will only result after a long measuring period.